1. Field of the Invention
The present invention relates to a measurement processing apparatus for automatically measuring a geometric feature of an image based on at least one measurement reference element set on the image. For example, the image may be a medical image, and the geometric feature may be a distance between measurement reference points, or an angle between lines determined by measurement reference points.
2. Description of the Related Art
Recently, the computed radiography (CR) has become commercially practical as a technique for obtaining a radiographic image having an extremely broad radiation exposure range, and is currently used for diagnosis in the medical field. The current CR systems use a stimulable phosphor (accelerated phosphorescent material), which stores a portion of radiation energy when the stimulable phosphor is irradiated with a radiation such as X rays, and emits accelerated phosphorescent light having energy corresponding to the stored radiation energy when excitation light such as visible light or infrared light is applied to the stimulable phosphor. That is, in the current CR systems, a radiographic image of an object such as a human body is recorded in a stimulable phosphor sheet, and the stimulable phosphor sheet is scanned with excitation light so that accelerated phosphorescent light is emitted from the stimulable phosphor sheet. Then, the accelerated phosphorescent light is optically read by a photomultiplier or the like so that a digital image signal is generated, Finally, based on the digital image signal, the radiographic image of the object is recorded as a visible image in a recording medium such as a film made of a photosensitive material or displayed by a display device such as a CRT monitor. Such CR systems are disclosed in, for example, Japanese Unexamined Patent Publication No. 56(1981)-11395 and U.S. Pat. Nos. 4,258,264, 4,387,428, 4,276,473 and 4,315,318 respectively corresponding to Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-104645, 55(1980)-116340, and 55(1980)-163472.
However, in some medical fields such as orthopedics, the radiographic images are not only observed, but also used for measuring a geometric feature such as a scoliotic curvature of a spine (by the Cobb method or the Ferguson method), a backward bending (kyphotic) index, an ulnar deviation, or a radial rotation. For example, as explained by Kazuo Hiroshima and Kazuo Yonenobu in “A Guide to X-ray Image Measurement in Orthopedic Surgery,” Kanehara Co., Ltd., 1990 (in Japanese), measurement of a geometric feature is conventionally made as follows.
First, measurement reference points are manually marked with a red pencil or the like on a film in which a radiographic image is recorded, and at least one geometric quantity such as distances between the measurement reference points or an angle between two lines connecting the measurement reference points is manually measured by using a measurement tool such as a ruler or protractor. Thereafter, a geometric feature such as a ratio between the distances, an area of a polygon determined by the measurement reference points, or the scoliotic curvature is obtained by calculation. That is, conventionally, geometrical analysis of a radiographic image is manually made.
For example, in the case of diagnosis of cardiomegaly, widths of a ribcage and a heart are measured in a radiographic chest image, and a diagnosis of cardiomegaly is made based on the cardio-thoracic ratio (i.e., a ratio between the widths of the heart and the ribcage). In this case, a doctor or a radiography technician is required to manually measure the widths of the ribcage and the heart by using a ruler or the like on the radiographic image, and calculate the cardio-thoracic ratio based on the measured values of the widths of the ribcage and the heart.
On the other hand, since radiographic images obtained by the CR systems are represented by digital image signals, it has been considered that if measurement of geometric features is automatically made, the load imposed on the doctor or the radiography technician during the measurement can be greatly reduced. That is, if geometric features to be measured (e.g., the scoliotic curvature or the cardio-thoracic ratio) and methods of measurement (e.g., calculation methods of the scoliotic curvature or the cardio-thoracic ratio) are stored in advance, and a radiographic image is displayed on a screen of an image display device such as a CRT monitor, and a user sets measurement reference points on the radiographic image on the screen, measurement of the scoliotic curvature or the cardio-thoracic ratio can be automatically made.
However, even when calculation of a geometric feature is automatically made as above, a user is still required to manually set measurement reference points on the screen, for example, with reference to a manual in which the methods of measurement or orders of setting of the measurement reference points are indicated. Therefore, conventionally, the efficiency of measurement of a geometric feature of an image and diagnosis based on the measurement is low.